1. Field of the Invention
The present invention relates to a process for the oligomerization of an acyclic monoolefin monomer and more particularly concerns the control of the oligomerization reaction to produce predetermined relative amounts of at least two preselected oligomers in a higher boiling fraction of the oligomerization products and the recycle of a lower boiling fraction of the oligomerization products.
2. Discussion of the Prior Art
It is known to recycle certain lower boiling fractions of oligomerization products to the reaction mixture for the oligomerization process in order to increase the yield of higher boiling oligomerization products. For example, European Patent Application No. 349276, filed on Jun. 27, 1989 and published on Jan. 3, 1990, discloses the essentially complete oligomerization of a C.sub.8 -C.sub.12 .alpha.-olefin and the subsequent removal of the dimer of the C.sub.8 -C.sub.12 .alpha.-olefin and unreacted C.sub.8 -C.sub.12 .alpha.-olefin monomer from the oligomerization product, followed by the separation of the trimer from the remaining oligomerization product and combination of a portion of the separated trimer with a C.sub.8 -C.sub.12 .alpha.-olefin in the reaction mixture in a subsequent oligomerization. The subsequent oligomerization essentially completely converts the C.sub.8 -C.sub.12 .alpha.-olefin and converts the low viscosity unsaturated trimer to an oligomerization product from which dimer and any unreacted C.sub.8 -C.sub.12 .alpha.-olefin monomer are removed. The remaining oligomerization product has a medium viscosity.
Nelson et al., U.S. Pat. No. 4,484,014, issued on Nov. 20, 1984, discloses a process in which a branched olefin mixture is oligomerized to produce a lube-oil range hydrocarbon mixture and in which unreacted branched olefins after the oligomerization step or olefins remaining after the lube-oil range hydrocarbon mixture has been separated from the oligomerization mixture may be recycled to the oligomerization step.
Nipe et al., U.S. Pat. No. 4,225,739, issued on Sep. 30, 1980, discloses that light boiling fractions of the oligomerization product are recycled to the oligomerization step in order to increase the yield of the desired heavier oligomerization product, hopefully without degrading the physical properties of the finished product. The patent reports that, in the oligomerization of short chain and long chain 1-olefins in the presence of Friedel-Crafts catalysts to produce super-quality synthetic lubricants, the use of recycle with catalysts such as aluminum chloride did increase the ultimate yield, but the physical properties were either unfavorable or showed little change compared with the properties obtained when recycle was not employed. By contrast, Nipe et al. discloses that when a boron trifluoride catalyst promoted by either water or alcohol was employed, the product yield was essentially unchanged from the yield when recycle was not employed, but the fluid viscosity decreased markedly accompanied by an improvement in viscosity index.
In particular, Nipe et al. discloses a process for producing lubricants having a low viscosity and a high viscosity index that comprises oligomerizing a mixture of short-chain 1-olefins, long chain 1-olefins, and low-boiling recycle from a previous oligomerization run in the presence of boron trifluoride catalyst promoted with water or alcohol. The recycle employed is obtained from a previous oligomerization run and is the overhead fluid fraction obtained when the total oligomerization product is topped to meet a flash point of at least 400.degree. F.
European Patent Application No. 95106749.5, filed May 4, 1995 and published Nov. 8, 1995, discloses a process for the oligomerization of alpha-olefin monomer in which the oligomerization product is fractionated and unreacted alpha-olefin monomer and a portion of the dimers and trimers in the product are separated and recycled to the oligomerization step. The patent application states that the use of recycle in the process results in an improvement in the yield of the higher oligomeric product and permits product properties to be adapted to the respective requirements.
However, none of the aforesaid prior art processes involve both the use of recycle and control of the oligomerization reaction to produce an oligomerization product mixture whose higher boiling fraction contains predetermined relative amounts of at least two preselected oligomers and has a predetermined set of properties such that it can be used directly in or as a synthetic lubricating oil. In fact, only European Patent Application No. 349276A2 discloses the chemical composition of the oligomerization product produced. Furthermore, typically the oligomerization product produced in prior art processes is fractionalized and the composition of the higher boiling fraction is adjusted by the addition or removal of components prior to its being used in or as a synthetic lubricating oil.
Thus, it would be highly desirable to employ recycle of a portion of the oligomerization product mixture and control of the oligomerization reaction in order to improve both the selectivity of the production of an oligomerization product mixture whose higher boiling fraction has a composition and set of properties such that the higher boiling fraction can be used directly in or as a synthetic lubricating oil.